Mitochondrial Zea mays Brittle1-1 Is a Major Determinant of the Metabolic Fate of Incoming Sucrose and Mitochondrial Function in Developing Maize Endosperms.

Abdellatif Bahaji,Jun Li,Jose María Seguí-Simarro,Alba Rivas-Sendra,Miroslav Ovecka,Edurne Baroja-Fernández,Verónica Parra-Vega,Ángela María Sánchez-López,Francisco José Muñoz,Carolina Camacho-Fernández,Javier Pozueta-Romero,Goizeder Almagro

doi:10.3389/fpls.2019.00242

Abstract

Zea mays Brittle1-1 (ZmBT1-1) is an essential component of the starch biosynthetic machinery in maize endosperms, enabling ADPglucose transport from cytosol to amyloplast in exchange for AMP or ADP. Although ZmBT1-1 has been long considered to be an amyloplast-specific marker, evidence has been provided that ZmBT1-1 is dually localized to plastids and mitochondria (Bahaji et al., 2011b). The mitochondrial localization of ZmBT1-1 suggested that this protein may have as-yet unidentified function(s). To understand the mitochondrial ZmBT1-1 function(s), we produced and characterized transgenic Zmbt1-1 plants expressing ZmBT1-1 delivered specifically to mitochondria. Metabolic and differential proteomic analyses showed down-regulation of sucrose synthase (SuSy)-mediated channeling of sucrose into starch metabolism, and up-regulation of the conversion of sucrose breakdown products generated by cell wall invertase (CWI) into ethanol and alanine, in Zmbt1-1 endosperms compared to wild-type. Electron microscopic analyses of Zmbt1-1 endosperm cells showed gross alterations in the mitochondrial ultrastructure. Notably, the protein expression pattern, metabolic profile, and aberrant mitochondrial ultrastructure of Zmbt1-1 endosperms were rescued by delivering ZmBT1-1 specifically to mitochondria. Results presented here provide evidence that the reduced starch content in Zmbt1-1 endosperms is at least partly due to (i) mitochondrial dysfunction, (ii) enhanced CWI-mediated channeling of sucrose into ethanol and alanine metabolism, and (iii) reduced SuSy-mediated channeling of sucrose into starch metabolism due to the lack of mitochondrial ZmBT1-1. Our results also strongly indicate that (a) mitochondrial ZmBT1-1 is an important determinant of the metabolic fate of sucrose entering the endosperm cells, and (b) plastidic ZmBT1-1 is not the sole ADPglucose transporter in maize endosperm amyloplasts. The possible involvement of mitochondrial ZmBT1-1 in exchange between intramitochondrial AMP and cytosolic ADP is discussed.

Highlights

Mitochondria are the main sites of cellular respiration and ATP supply
Our findings show that mitochondrial Zea mays Brittle1-1 (ZmBT1-1) is a decisive factor in primary metabolism and mitochondrial function in developing maize endosperms and raise important questions regarding the role of BT1 in cereal endosperms
The level of expression of Cell wall invertase 2 (CWI-2) is greatest early during seed development and it drops from the 12 days after pollination (DAP) stage (Cheng et al, 1996; Prioul et al, 2008) whereas SH1 expression increases from the 14 DAP developmental stage (Doehlert et al, 1988; Méchin et al, 2007; Prioul et al, 2008; Li et al, 2013)

Summary

Introduction

Mitochondria are the main sites of cellular respiration and ATP supply They play important roles in diverse processes such as redox homeostasis and provision of molecules that act as metabolic intermediates in essential biosynthetic pathways or as specific signals that modulate nuclear-encoded protein expression (Chandel, 2014). MCF proteins are involved in the transport of nucleotides, phosphate, di- and tri-carboxylates, amino acids, and cofactors across the mitochondrial membrane. They are all presumed to be targeted to the mitochondrial inner membrane, some of them have been shown to occur in peroxisomes, glyoxysomes, plasma membrane, and plastids (Sullivan and Kaneko, 1995; Fukao et al, 2001; Bedhomme et al, 2005; Palmieri et al, 2006; Bahaji et al, 2011a,b). ZmBT1-1 is present as three 39, 40, and 44 kDa proteins (Cao et al, 1995; Sullivan and Kaneko, 1995), the former two being processing products generated within the plastidial compartment (Li et al, 1992)

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